Production of Sponge Iron by Oxidation Roast and Carbothermic Reduction of Ok Tedi Pyrite concentrates

Transcription

1 7 th HUON SEMINAR ACHIEVING VISION 2050 THROUGH HIGHER EDUCATION, RESEARCH, SCIENCE & TECHNOLOGY November 13 th to 14 th 2013, Papua New Guinea University of Technology, Lae, Papua New Guinea HS Production of Sponge Iron by Oxidation Roast and Carbothermic Reduction of Ok Tedi Pyrite concentrates M. Kama 1, A. K. Chakrabarti 2 and P.S.A. Leki Mining Engineering Department, PNG Unitech. s: 1 mkama@minining.unitech.ac.pg; 2 akc1940@gmail.com ABSTRACT The OK Tedi mine in the Western Province has constructed a pyrite flotation plant at Mt. Fubilan to concentrate the pyrite from the copper plant tails. The pyrite concentrates are transported in a slurry form to Bige, where it is buried below the water table to prevent oxidation and generation of acid mine drainage. This study was done to assess the possibility of converting the pyrite concentrate into sponge iron for use in steel plant. Approximately 10 grams of pyrite concentrate from OK Tedi mine with bulk chemical composition (%) of C= 11.8, O=23.5, Mg=0.07, Al=1.2, Si=4.1, S=28.1, K=0.6, Ca=0.7 and Fe=29.6 (SEM EDAX analysis) were roasted in a muffle furnace at 700 o C, 750 o C and 800 o C respectively, for periods ranging from 5 mins to 40 mins. The weight change after roasting of pyrite was recorded and it was noticed that the roasting kinetics are marginally faster at 800 o C. For the carbothermic reduction experiments, the pyrite concentrate of 10grams was roasted at at 800 o C for 1 hour. The sinter was mixed thoroughly with 30% graphite and was further reduced at 1100 o C for periods ranging from 30 mins to 120 mins. Sponge iron of low degree of metallization was produced after 120 minutes. Composition in (%) of the reduced sinter by SEM EDAX show C=4.7, O=33.6, Mg=0.7, Al= 0.7, Si= 4.4, S =4.5, Ca= 0.7, Fe= 5.7. Oxygen was not completely removed, but the iron content increased appreciably due to removal of sulphur. The results suggest that the pre-roasting of the pyrite concentrate needs to be carried out at a higher temperature than 800 o C to remove sulphur completely. It also suggests that the final reduction temperature needs to be below 1000 o C because CO 2 is unstable above this temperature. Therefore, it is necessary to adjust the temperature and time of reduction roasting to prevent re-oxidation of the sponge iron produced. Reduction of iron oxide will occur more efficiently if the initial sulphur in the charge is low. Hence magnetic separation of the iron oxides may be tried to increase grade. INTRODUCTION The sulphide copper ore of Ok Tedi mines contains considerable amount of pyrite (FeS 2 ). The pyrite usually passes into the tailing. The tailing is separately treated to float up the pyrite. At the moment the pyrite is stored below water table in Bige. Thus, a large quantity of pyrite is accumulating at Bige which is located almost 20km from the mine site. The present investigation was carried out to explore the feasibility of recovering the iron value in pyrite in the form of sponge iron. Sponge iron has emerged as an important feed material for steelmaking in many countries. Hence economic production of sponge iron 47

2 will not only solve the problem of disposal of huge quantities of pyrite from the mine site but also open up the prospect of setting up small scale electric arc or induction furnace based steel plants in PNG. REVIEW OF LITERATURE Pyrite may be oxidized either to iron sulphate or iron oxide. However, conversion of pyrite to ferrous sulphate occurs at low temperature ( C) only [1]. During roasting at high temperatures (900 C) iron sulphide is converted to iron oxide [2]. Apart from thermo-gravimetric techniques like DTA and DTG, advanced techniques like Mossbauer spectroscopy and magneto-kinetic measurements have been employed to quantitatively assess the kinetics of roasting of pyrite [3-8]. Isothermal carbothermic reduction of iron oxide has been investigated extensively in the past. Techniques used included reduction of carbon coated pellets of finely ground ore, reduction of ore charcoal pellets and fluidized bed reduction of iron ore powders in hydrogen or carbon monoxide [6-8]. EXPERIMENTAL PROCEDURE The pyrite concentrate was analyzed by X-ray diffraction and energy dispersive analysis in JEOL scanning electron microscope. The analysis revealed that in addition to FeS 2, it contained carbonaceous matter and oxides of Al, Mg, Si and Fe. The pyrite was first roasted isothermally at comparatively low temperatures of 700 C, 750 C, & 800 C respectively for periods ranging from 5 minutes to 40 minutes in order to monitor the oxidation reaction by weight change measurements. The pyrite was converted to iron oxide. No attempt was made to recover SO 2, as the main thrust of the present investigation was on the recovery of iron. In the second step, adequate quantity of pyrite was first roasted at 1100 C for one hour. The objective of high temperature roasting was to make sure that the sulphide is converted to oxide completely. Then the roasted product was reduced by carbon at 1100 C for varying periods of time. In each crucible 50 grams of roasted oxide was mixed with 30wt% charcoal. The crucibles were heated isothermally at 1100 C for periods ranging from 30 minutes to 120 minutes. Weight loss during the roasting operation was carefully recorded. The initial roasted sample and the one reduction roasted for 120 minutes were examined in XRD. SEM EDAX analysis was also conducted on the samples reduction roasted for 90 minutes and 120 minutes. RESULTS AND DISCUSSIONS The XRD patterns of the pyrite concentrate is given in Figure 1. 48

3 Counts 6000 Sample-2 Al2 O3; 4000 Al2 O3; 2000 Al2 O3 Al2 O3; Al2 O3; Al2 O3; Fig. 1 XRD pattern of pyrite concentrate. Position [ 2Theta] The EDAX analysis of the pyrite concentrate is given in Table 2. Table 2. EDAX analysis of the pyrite concentrate. Element Wt% C O Mg 0.69 Al 1.19 Si 4.14 S K 0.59 Ca 0.69 Fe Although the XRD pattern suggests that the main gangue material in pyrite is A1 2 O 3, it may be reasonably suspected from the EDAX data that considerable quantity of iron oxide remains mixed up with the pyrite. The large proportion of carbon in the pyrite concentrate is interesting. It is quite likely that the vegetable matter present in the ore is also floated with the concentrate. During oxidation roasting, the expected reaction is 4FeS = 2Fe 2 O 3 + 8S0 2. The pungent smell of SO 2 gas could be noted during the experiment. The S0 2 gas was allowed to escape, and there was no reduction in mass of the charge after roasting. The degree of conversion ( ) was calculated as follows:- 49

4 = W o W t W o where W o = initial weight of charge W t = weight of charge at time t Fig. 2. Degree of conversion is plotted against time. Fig. 3. Corresponding weight loss (as S0 2 ) with time. 50

5 It is clear from these figures that an increase in the roasting temperature from 700 C to 800 C had only a small effect on the extent of oxidation in the initial period. After 30 minutes there was no change in the degree of conversion at any temperature in 700 C to 800 C range. It therefore appears that for conversion of sulphides to oxides, roasting needs to be carried out at a much higher temperature. It is however not clear how the carbonaceous matter affected the roast reaction. For the purpose of reduction roasting adequate quantity of the pyrite concentrate was first roasted at 1100 C for 1 hour. Then, 50 grams of the roasted sample was mixed with 30wt% charcoal (15grams) in each of four crucibles. After heating the charge isothermally at 1100 C, the crucibles were withdrawn from the furnace at intervals of 30 minutes, cooled down and weighed again to record the change in weight of charge. The data are presented in Table 3. Table 3. Result of reduction roasting. Serial No Time of isothermal roasting (minutes) Initial weight of charge (roasted pyrite 50gm + charcoal = 15gm) Final weight of charge (grams) Weight (grams) charge The above results suggest that substantial weight loss occurs after 30 minutes roasting. The weight loss may be linked to oxidation of carbon and sulphur and reduction of oxides in the charge. But it is interesting to note that the extent of weight loss decreased initially up to 90 minutes. After 120 minutes roasting, there was a net weight gain. This is probably an indication of re-oxidation after 30 minutes of roasting. It is well known that CO 2 is not stable above 1000 C. Hence the p net reaction may be summarized as Fe (s) + C(s) = Fe(s) + CO(g). However, actual reduction of Fe to Fe takes place in several stages. Simultaneously, any CO 2 formed is quickly converted at C to CO by the solution loss reaction CO 2 (g) + C(s) = 2CO(g). Direct oxidation of the carbon to carbon monoxide is also possible within the oxidizing atmosphere of the furnace. Therefore both direct reduction by carbon and indirect reduction by carbon monoxide are possible at 1100 o C. The EDAX analysis of samples reduced for 90 minutes and 120 minutes at 1100 o C are compared in Table 4. XRD patterns of the pyrite roasted at 1100 o C for 1 hour and also for the one reduction roasted for 120 minutes are given in Figure 4 & 5 respectively. Table 4. SEM EDAX analysis of roasted products. Element wt% 90 minutes 120 minutes C O Mg Al Si S K Ca Fe

6 Counts 6000 sample-3 ; ; Fe3 O Fe3 O4; Fe S; Fex-1 Sx Fe S; Fex-1 Sx Fe S; ; Fex-1 Sx ; Fex-1 Sx ; ; Al2 O3 ; Fe3 O4; Fe S ; ; Al2 O3 ; ; Fex-1 Sx; Al2 O3 Fe S ; Fex-1 Sx; Al2 O3 ; Position [ 2Theta] Fig. 4. XRD pattern of pyrite reduction roasted at 1100 o C for 90 min. Counts 1000 sample-4 Fe3 O4 ; Fe5 C2 ; Fe3 O4; Fe5 C2 Fe5 C2 Fe; ; Fe5 C2 Fe3 O4 ; Fe3 O Position [ 2Theta] Fig 5. XRD pattern of pyrite reduction roasted at 1100 o Cfor 120 minutes. 52

7 The EDAX analysis results may be matched with the XRD patterns given in figure 4 for the roasted product (1100 o C/1 hour). Figure 4 clearly indicates that various iron sulphide phases are still present along with the oxides, mainly Fe and Fe When this incompletely oxidized product is reduction roasted at 1100 o Cfor 120 minutes, the emergence of iron (Fe) phase could be detected in figure 5. But in conformity with the EDAX analysis results, the XRD data also confirm that oxides such Fe and Fe were also present in considerable quality. In addition, a carbide phase (Fe 5 C 2 ) had also formed. Indeed the pellet collected after reduction roasting for 120 minutes had a sponge iron like appearance. SUMMARY The real challenge therefore is to increase the degree of metallization. This limited experiment however, has provided the following information:- 1) Effective oxidation roasting of the pyrite may be carried out at 1100 o C. However a prolonged treatment may be necessary to completely convert the sulphides to oxides. 2) Reduction of iron oxide will occur more efficiently if the initial sulphur in the charge is low. Hence magnetic separation of the iron oxides may be tried. 3) It is necessary to adjust the temperature and time of reduction roasting to prevent re-oxidation of the sponge iron produced. ACKNOWLEDGMENT Pyrite concentrate was obtained as a bye product from the concentrator in Ok Tedi Mining Ltd. The authors thank Ok Tedi for their help. Many thanks also to NALTS, for provision of experimental facility. REFERENCES 1. Embaic A. Ferrow, Mannerstand Mania, Sjoberg Bosser. Reaction kinetics and oxidation mechanics of the conversion of pyrite to ferrous sulphate. A Mossbauer spectroscopy study, Hyperfine Interaction, volume 163/1-4 (2005), pp T. Rosenqvist, Principles of extractive Metallurgy, McGraw Hill International Book company, Inter. Student Edition, (1983), PP Devia M. wilkomirsky I, Parra R., Roasting Kinetics of high arsenic copper concentrates A review, Minerals and Materials processing, 29/2 (2012), pp Prasad A, Singru R. M, Biswas A.K, Physica status solid, Study of roasting of pyrite minerals by Mossbauer spectroscopy 87/1 (1985), PP Marusak L. A, Walker P, Mulay L.N, The magnetokinetics of oxidation of pyrite (FeS 2 ), Magnetics, IEEE Trans. 12/6, (1976) PP Baliarsingh SK., Mishra B. B. Tech. Thesis, NIT ROURKELA India

8 7. Sarkar S.B, Ray H.S, ChatterjeeI. Kinetics of reduction of iron ore coal pellets, Jr. of Thermal Analysis 35/7, (1989) PP Bohn C.D, cearton J.P, Muller C.R. Davidson J. F, Mayhurst A.N, Scot S.A, Dennis J.S, The Kinetics of the reduction of iron oxide by carbon monoxide mixed with carbon dioxide A.l.ch. E Jr., 56/4 (2010), PP